NFS: Fix I/O request leakages

When we fail to add the request to the I/O queue, we currently leave it
to the caller to free the failed request. However since some of the
requests that fail are actually created by nfs_pageio_add_request()
itself, and are not passed back the caller, this leads to a leakage
issue, which can again cause page locks to leak.

This commit addresses the leakage by freeing the created requests on
error, using desc->pg_completion_ops->error_cleanup()

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Fixes: a7d42ddb ("nfs: add mirroring support to pgio layer")
Cc: stable@vger.kernel.org # v4.0: c18b96a1: nfs: clean up rest of reqs
Cc: stable@vger.kernel.org # v4.0: d600ad1f: NFS41: pop some layoutget
Cc: stable@vger.kernel.org # v4.0+